The present invention relates to a method of feedback-controlling the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine, and more particularly to a method of this kind which is adaped to properly control the air-fuel ratio when the engine is accelerated under high load.
An air-fuel ratio feedback control method for internal combustion engines is known, e.g. from Japanese Provisional Patent Publication (Kokai) No. 62-157252 owned by the assignee of the present application, in which the air-fuel ratio of an air-fuel mixture being supplied to an internal combustion engine is controlled by the use of a correction coefficient varying in response to the output of an exhaust gas ingredient (e.g. oxygen) concentration sensor arranged in the exhaust system of the engine during operation in a predetermined air-fuel ratio feedback control region. An average value of values of the correction coefficient assumed during engine operation in the predetermined air-fuel ratio feedback control region is calculated and applied to air-fuel ratio feedback control which is carried out later.
According to the above proposed method, the feedback control region is divided into an idling region and regions other than the idling region (hereinafter called "off-idling region") including an accelerating region in which the engine is accelerated from the idling region to start the vehicle from a standing state (standing-acceleration). Average values of the correction coefficient are respectively calculated during engine operation in these divided regions and stored. The stored average values are applied in place of the correction coefficient when the operating region of the engine has shifted to the respective regions. The application of the average value in the above accelerating region is intended to prevent the air-fuel ratio of the mixture from being leaned at the standing-acceleration of the engine, due to adhesion of fuel injected from fuel injection valves of the engine to the inner wall surface of the intake pipe at the start of standing-acceleration, to thereby reduce the amount of emission of NOx.
Although the proposed method can prevent leaning of the air-fuel mixture at the time of standing-acceleration, it shows the following disadvantages if it is applied when the engine is accelerated under high load within the feedback control region, e.g., when the driver accelerates the engine in order to pass a vehicle during running of the vehicle (high load acceleration).
Also at high load acceleration of the engine, especially in the case that the accelerator pedal is abruptly stepped on, fuel becomes stuck to the inner wall surface of the intake pipe to cause leaning of the air-fuel ratio. In the feedback control according to the conventional method, the amount of fuel being supplied to the engine is increased or decreased at a relatively small rate by integral control, so long as the output of the exhaust gas ingredient concentration sensor remains on the same side with respect to a predetermined reference value, i.e. either smaller or larger than the predetermined reference value, that is, until it changes across the predetermined reference value. Consequently, the air-fuel ratio continues to be lean for a long time during the high load acceleration. Therefore, an increase in the emission of NOx cannot be prevented.
Further, by effecting the feedback control during the high load acceleration as above, the correction coefficient is varied in such a direction as to enrich the air-fuel ratio, and an average value of the correction coefficient for the off-idling region is calculated by the use of thus varied values thereof. As a result, the calculated average value is accordingly varied in such a direction as to enrich the air-fuel ratio. Consequently, if the average value is applied when the engine has shifted to a condition other than the high load accelerating condition, e.g. a steady condition in which the vehicle is cruising, the air-fuel ratio is enriched, resulting in increased emission of HC and CO. Thus, according to the above conventional method, if the high load acceleration is frequently carried out, the emission characteristics of the engine can be degraded, not only when the engine operating condition is shifted to the high load accelerating condition, but also when it is shifted e.g. to a steady condition in which the vehicle is cruising.